I was recently e-mailed the following question:

Is there any risk that modern medicine is diluting the strength and stability of human beings by essentially taking natural selection out of the picture? By entertaining a desire to prolong individual lives, are we propagating genetic weaknesses (“weaknesses” in the Darwinian sense) that could ultimately lead to the demise of humanity, or is that such a potentially lengthy process that humanity as we know it will likely have evolved into something new or become otherwise extinct before that could ever occur? I hope you understand that I am not necessarily advocating that we allow people with genetic weaknesses to die so that they might not procreate and dilute my gene pool. I was just wondering if there’s any validity to this thought.

This is a common question and the answer has several components.

Genetic Diseases

It makes a sense that inherited diseases would be selected against and would die out naturally over time, and if we artificially keep the victims of genetic diseases alive then we are keeping those genes alive to propagate throughout the population. But a closer look at the details reveals a much more complex picture, and one that mostly contradicts first impressions.

There are different kinds of genetic diseases – for simplicity I will consider just two main categories: dominant and recessive. We have two copies (alleles) of every gene, one from each parent. Dominant traits are those that are expressed to some degree in everyone who has one copy of the disease trait. Recessive disorders require two copies; someone who has just one copy is a carrier and will not manifest the disease. (I am simplifying, there are exceptions and complexities to all of this, for example a dominant trait may have incomplete expression and a recessive carrier may have some manifestations.)

For recessive genes the number of people with the disease is vastly outnumbered by the number of carriers. Depending upon how rare the recessive allele is, there may be thousands or even millions of carriers for every afflicted person with a double dose of the recessive allele. So removing the affected individuals from the breeding population has an insignificant effect on the frequency of the recessive allele in the population.

Dominant genetic diseases are different – every person carrying even one allele with the disease variant will have the disease. So here we must further ask how and when the disease manifests. If it manifests after the age at which most people have their children, then it won’t necessarily be selected against anyway. If it manifests in childhood and is very severe, even fatal, despite modern medicine, then medicine won’t be salvaging the disease allele. But if we can keep children with the dominant disease alive long enough to have children of their own, we will be increasing the frequency of the disease in the population.

However, in such cases people know they have the disease. There are no carriers – everyone with the gene has the disease and knows it. This means that they can decide not to have children in order to avoid passing on their dominant gene. Other options will also like become available in the future – like filtering out sperm that carry the dominant gene prior to artificial insemination. We may also be able to perform DNA screening on fetuses to see if they are affected – although this is certainly going to be a hugely controversial procedure if it is used to decide whether or not to terminate a pregnancy. Such is already the case with chromosomal abnormalities. So the bottom line is that with dominant diseases modern medicine cuts both ways.

Also, we have to consider the spontaneous mutation rate. All genetic diseases have a spontaneous rate at which they pop up anew in the population. So even if we magically removed all genetic diseases from the population, they would reappear over time by mutation. Eventually such mutations reach a steady-state in the population. Improving the survivability may increase the steady state of a genetic disease, but then genetic testing and counseling decreases the steady state of a genetic disease. The latter effect may be greater than the former – which means that modern medicine may actually decrease genetic disorders, not increase them.

Genetic Predisposition

A separate situation to consider is diseases that are not genetic but for which there is a genetic predisposition – for example, heart disease. Here I think the effect of modern medicine does not weaken the population because mostly we are extending lives far beyond the reproductive years – years that are “invisible” to selective pressures. (There may be some benefit or detriment that older ancestors have upon their offspring. For example, having grandma around may have a survival benefit. But let’s put that aside for now.) Dying at 50 of a heart attack, after having all your children, would do nothing to remove the genes that predispose to heart attack from the population.

Diseases that are not genetic and yet afflict children or young adults are likely more the result of bad luck and bad genes. Overall health is also determined by environmental and behavioral factors, such as nutrition and exercise.

But there are clearly diseases that have a genetic predisposition that can strike people down in their youth and yet we keep them alive with modern medicine and this allows them to reproduce. However, modern medicine is not yet sufficiently powerful to completely remove the survival advantage of being genetically healthy. And when it is, being genetically healthy will no longer matter.

The Future

When asked to make a prediction for the next 50 years, Steven Pinker wrote: “Predicting the future is an invitation to look foolish.” I agree, but I’m going to do it anyway – although I will use broad brushstrokes to obscure my foolishness.

My prediction is this – before modern medicine has any significant net effect on the genetic health of the human population (and I am not acknowledging that it does, if all factors are taken into consideration), we will have the technology to take control of our own genetic destiny. We will, in effect, take control of our own evolution. Genetic engineering is already possible. There is no scientific or technological reason we cannot apply it to ourselves. There are obvious ethical and moral concerns, and we are already hearing from those who think that human cloning is evil or unnatural, but the history of such technology is that people eventually get used to it and it becomes less controversial over time.

Imagine, for example, if parents were given the option of choosing which of their alleles they contribute to their offspring, rather than leaving the entire process up to chance. At first this will be done to block the inheritance of genetic diseases. But this same technology could be used to determine eye color. The result will be “designer children.”

The next step will be, not just selecting genes, but altering them. Why should your children suffer because of your genetic failings? If you could give them a gene variant known to be associated with a lower risk of heart disease, why deprive them of it simply because you don’t have that variant in your own genome. If you can give them a gene to reduce disease risk, then what about enhanced intelligence, or strength, or an optimized immune system, etc. Imagining such things for our own children is different than contemplating them for society. Of course, there will be purists of various stripes. But those with money and a desire for perfect children are likely to drive such technology forward.

If we keep extrapolating we end up in the science fiction futures where humanity is barely recognizable. There are too many unknowns between here and there to paint a picture of such a future with any confidence, but I think the broad brush strokes are likely accurate. We will take charge of our genetic future and natural evolution will be replaced by artificially engineered evolution.